It is known to provide a color image having a gradation in the cross-web direction by coating techniques such as gravure printing. Products having such a variation find utility in packaging applications as well as in safety glass for automobiles and in architectural applications.
In some applications, a gradated colored stripe is formed by coextruding a molten colored layer along with a clear thermoplastic sheet. In this process, the colored stripe is embedded in and forms an integral part of the thermoplastic sheet. U.S. Pat. No. 4,316,868 describes one such process.
These processes have inherent limitations. For example, in applications such as gravure printing, the overall quality of the gradated image is dependent on the quality of the engraved gravure cylinder. Further, applications which utilize pigment-based coating solutions place additional limitations on gravure printing due to accumulation of pigment particles under the doctor blade which is used in gravure printing. These particles cause streaks in the coated product which make it unacceptable for applications requiring high optical quality.
While the coextrusion process overcomes the quality limitations inherent in gravure printing, flexibility with respect to colors and widths of the gradations is limited. Further, color and width changes require extensive cleaning of the die and modifications to the die assembly which are costly in terms of down-time and labor requirements involved in the operation.
Slot die extrusion coating of uniform coating thickness in the cross-web direction has been described in U.S. Pat. No. 4,411,614 wherein inserts in flow distribution channels are used to improve the flow uniformity of coating solutions in the cross web direction. The inability of this method, however, to produce an optical density gradient in the cross-web direction precludes this process from being used in producing colored windscreens.
Slot die coatings of variable thickness has been described in Patent Application PCT/US91/06899. This process, however, requires a symmetrical/mirror image design which minimizes its utility for colored windscreen applications.
In the laminated safety glass industry, especially laminates for automotive windscreens, the use of automated unwinding, stretching and blanking equipment is widespread. For this equipment to work properly, o especially with gradient band interlayers for automotive windscreens, the interlayer roll and the orientation of the gradient band on that roll should be constant. That is, for example, the roll should always unwind clockwise or counterclockwise and the colored band should always be oriented up or down on the roll.
With a symmetrical/mirror image coating design as described in Patent Application PCT/US91/06899, only one slit band of a set will yield on transfer an interlayer roll with the desired orientation. The other slit band will yield a roll of unacceptable, opposite orientation.
It has been shown that if the unacceptable slit roll of coated carrier is first rewound backwards, it too will yield the proper orientation on transfer. However, the re-winding operation is costly and reduces quality by subjecting the coated carrier to possible physical damage as well as the introduction of contaminants which would make it unacceptable for high optical requirements like automotive windscreens.
Further, the necessity to make a set of two bands restricts flexibility to meet marketplace requirements since many different bandwidths exist. If maximum width is produced, substantial waste is created when smaller bands are called for.
The limitations associated with the coating processes heretofore used can be overcome by coating an asymmetric, non-mirror image design, i.e., a band or bands that vary in thickness and width, if desired, in the cross-web direction as measured from right-to-left or left-to-right. Upon slitting, all bands will immediately have the same, desired orientation and width.
The use of an asymmetric shim design for coating the composite layers provides improved process utility in that all the bands will immediately have the same winding orientation when slit, thereby, eliminating damage and contamination concerns.
A further advantage of the asymmetric shim design is evident in that one can produce any number of desired bands or bandwidths within a given design subject only to the physical dimensions of the die body.
Because of the difference in coating thickness at the edges of an asymmetric shim design, coating stability can be adversely affected. It has been found that the addition of a dummy band, that is, a narrow band of equivalent maximum coating thickness to the opposite edge, stabilizes the coating process. This narrow band is discarded after slitting. The width of the band is chosen to maximize stability but minimize slit waste.
U.S. Pat. No. 5,500,274, filed Mar. 22, 1994, discloses a gradated color strip process by coating the stripe on a carrier film with subsequent transfer to the thermoplastic sheet. This process creates unstable process control of the variable thickness coating bead due to processing conditions such as web speed, solution viscosity, and coating thickness.
The use of a vacuum below the coating bead to counter the drag of the moving web and stabilize the coating bead is known. In general, the vacuum applied through a slot is uniform across the web. U.S. Pat. No. 4,265,941 discloses the use of a segmented slot to control the formation of thick regions of the coating at the edges of the carrier film which are typically called edge beads.
Herein, however, it is shown that varying the applied vacuum in the cross-web direction by variably plugging the vacuum slot, dependent on a given shim design, enhances the coating process by optimizing the vacuum level across the web and, thereby, providing coatability over a greater vacuum range, herein defined as vacuum latitude.